1. Field of the Invention
The present invention is related to the field of surveying of wellbores. Specifically, the present invention uses 3-component magnetometer and accelerometer data downhole to determine an azimuth of the borehole relative to magnetic north and then uses a determination of magnetic declination using Global Positioning Satellite (GPS) data to obtain an azimuth relative to true north.
2. Description of the Prior Art
Surveying of wellbore orientation is commonly performed by the use of instruments containing sets of three orthogonal axes accelerometers and magnetometers, which are inserted within the drillstring and used to measure the orientations of the local gravitational and magnetic field vectors. In order to measure the earth""s magnetic field, which is used as a north reference from which wellbore azimuth may be computed, the instruments must be placed within a section of non-magnetic material extending between upper and lower ferromagnetic drillstring sections. These ferromagnetic portions of the drillstring tend to acquire magnetization as they are repeatedly strained in the earth""s magnetic field during drilling operations. The nominally non-magnetic portion of the drillstring may also acquire some lesser magnetization as a result of imperfections. The result is that magnetometer measurements made by an instrument within a drillstring may measure not the undisturbed magnetic field, but the vector sum of the earth""s field and an error field caused by drillstring magnetization. Since the tool is fixed with respect to the drillstring, the error field is fixed with respect to the tool""s coordinate system and it appears as bias errors on the magnetometer measurements, which can lead to errors in the determination of wellbore azimuth and trajectory unless measures are taken to compensate for these bias errors.
Since the greater part of the drillstring magnetization occurs in the ferromagnetic portions of the drillstring, which are displaced axially from the instrument, the bias error in the axial direction usually exceeds the transverse bias errors. Various methods have therefore been published which seek to determine axial magnetometer bias errors in a single directional survey, including U.S. Pat. No. 3,791,043 to Russell, U.S. Pat. No. 4,163,324 to Russell, Re. U.S. Pat. No. 33,708 to Roesler, U.S. Pat. No. 4,761,889 to Cobern, U.S. Pat. No. 4,819,336 to Russell, U.S. Pat. No. 4,999,920 to Russell, and U.S. Pat. No. 5,155,916 to Engebretson. All of these methods require the provision of an independent estimate of one or more components of the earth""s magnetic field, and as a result all of them tend to lose accuracy in those attitudes in which the direction of the independent estimate is perpendicular to the drillstring and therefore contributes little or no axial information. In particular, all of these methods lose accuracy as the wellbore attitude approaches horizontal east-west. A number of methods have also been published which seek to determine magnetometer biases on all three axes, including U.S. Pat. No. 4,682,421 to van Dongen and U.S. Pat. No. 4,956,921 to Coles, and UK Pat. No. 2,256,492 to Nicolle. While certain of these methods can resolve transverse bias components without external estimates of the field, they all require an independent estimate of the earth""s magnetic field in order to determine the axial bias component, and therefore they also tend to lose accuracy as the attitude approaches horizontal east-west. U.S. Pat. No. 4,709,486 to Walters discloses a method for determining axial bias errors without any external estimate, by the simultaneous use of transverse magnetometer data from a plurality of surveys. However the method fails to make use of the valuable information contained in the axial magnetometer measurements, since it does not require any correlation between the axial biases determined at the plurality of attitudes. In U.S. Pat. No. 5,321,893, Engebretson discloses a method which may be used to determine magnetometer scale factor and bias errors from a plurality of surveys with or without requiring any external estimate of the earth""s field. However, the method is inherently approximate since it requires the construction of a xe2x80x9cmeasurement matrixxe2x80x9d, whose elements depend on the unknown borehole attitude and magnetic dip angle. U.S. Pat. No. 5,623,407 to the present inventor and having the same assignee discloses a method for determining magnetometer biases during wellbore survey operations, which is capable of determining biases on up to three axes, with or without the use of an external estimate of the local magnetic field, and which is capable of providing an accurate result using data from a minimum number of surveys. Also disclosed in U.S. Pat. No. 5,623,407 is a method for determining magnetometer biases which may vary between surveys in a predefined manner. What is lacking in prior art is the ability to deal with biases in the accelerometers and properly correcting for them,. the ability to estimate the uncertainty of corrected measurements and the ability to acquire and use locally measured reference values of the earth""s magnetic field.
Additional objectives, features and advantages of the present invention will be apparent in the written description which follows.
The present invention provides a method for determining magnetometer errors during wellbore survey operations and referencing the corrected azimuth measurements to true north. It is capable of determining errors on up to three axes, with or without the use of an external reference measurement of the local magnetic field, and is capable of providing an accurate result using data from a minimum number of surveys. A model is used to correct the observed data and the corrected data are transformed from the tool coordinate system to a different coordinate system referenced to the earth. The difference between the corrected transformed data and reference data in the earth coordinate system is minimized to determine the model parameters. The present invention also provides a method for determining residual uncertainty in the measurements and for quality control of the measurements. By making the observations over a period of time, any deterioration of the sensors may be identified. When combined with a determination of magnetic declination using GPS data, a more accurate survey of a borehole relative to true north is obtained.